1. Field of the invention
These devices are piezoelectric actuators which are driven by the cross coupling of optically produced voltages (photovoltages) and piezoelectric constriction (inverse piezoelectric effect). This cross coupling is useful only if a strong piezoelectric and photovoltaic response is available in the same material. One type of material which has both of these properties are photovoltaic ferroelectric ceramics. Because of the anomalous photovoltaic properties these materials, a high length dependent voltage appears across an illuminated element of this material. This voltage then produces a strain in the material (or stress if the element is mechanically clamped). The strain is the result of the inverse piezoelectric effect: a voltage applied to a piezoelectric material induces a strain in the material. Photovoltaic ferroelectric ceramics, in addition to anomalous high photovoltages, are characterized by strong piezoelectric coupling and exhibit a strong inverse piezoelectric effect.
2. Description of the prior art
Thin cadmium sulfide wafers, having an orientation perpendicular to the crystallographic axis (001), when fastened at one end and exposed to illumination in the visible spectrum, exhibit pronounced bending. The reason given for this is that the light modulates the electric field at the surface (surface barrier) and causes an elastic strain due to the converse piezoelectric effect. See Lagowski et al, "Photomechanical Effect in Noncentrosymmetric Semiconductors-CdS," Applied Physics Letters, Volume 20, Number 1, Jan. 1, 1972.
In U.S. Pat. No. 4,252,440 issued Feb. 24, 1981 to Fedors et al, entitled "Photomechanical Transducer," light absorbing ultra-thin films mounted under a fixed strain exhibit the behavior of an optomechanical or photomechanical transducer. The transducer responds to light in a quick and reversible manner converting a time-variable light source into a time-variable mechanical stress.